The pressure within the eye has long been regarded as a significant clue in assessing the health of the eye. Physicians in Egypt and Greece, our earliest recorded civilizations, have regarded the hardness or pressure of the eye as being indicative of vision threatening eye disease. Initially, pressure was assessed with digital applanation. By this technique, the eye was balloted gently between opposing fingers of the left and right hand, and judgment was made by the clinician as to normal or abnormal pressure. Digital tonometry continued in practice throughout the early evolution of ophthalmology. Modern ophthalmic surgeons are familiar with the application of digital tonometry and can quite readily tell the difference between normal or abnormal pressures of moderate magnitudes.
The present uses of tonometry respect and honor age old traditions of digital tonometry. It is first and foremost important to know whether the pressure is normal or whether the pressure is significantly out of range from normal. This relates to both high and low pressures. However, the accumulation of accurately measured serial data over a patient's ocular history allows trending of pressures and frequently accurate prediction of certain pressure related events which may be vision threatening. Intraocular pressure is important in the differential diagnosis of acute eye problems. It is often important to measure the intraocular pressure in an eye which otherwise may have acute inflammatory, viral, bacterial, ischemic, or traumatic etiologies. Intraocular pressures also need to be measured in the immediate post-operative period when intraocular contamination is at greatest risk. A sterile instrument is thus desirable. Careful slit lamp biomicroscopic observation during the acquisition of applanation tonometry also allows a direct view of the intraocular pulse through the mires of the applanation device. This observation has significance in accessing carotid and intracranial blood flow and ocular perfusion.
The evolution of the ophthalmic subspecialty as a scientific as well as an artful practice, however, required standardization and quantification of these measurements. Consequently, one of the first biometric parameters to be utilized was tonometry. Schiotz tonometry is a mechanical, non optical gravity dependent device which measures intraocular pressures by using a metallic weighted probe to indent the cornea with the patient in a supine position. Although very small variations in weight are significant, accuracy has been found to be reproducible. Intraocular pressure curves and distributions of intraocular pressure within normal populations have been formulated. Normal and abnormal ranges and standards have been identified. This important biometric data accumulation has founded the basis for investigation into the etiology and treatment of glaucoma, a disease whereby the intraocular pressure damages the optic nerve.
The means for increased convenience of obtaining this measurement followed the introduction of the slit lamp biomicroscope and, additional forms of pressure measurement were developed. In particular, what is known in the profession as the Goldmann applanation tonometer was designed for use at the stereo, slit lamp biomicroscope for pressure measurement convenient and comfortable to both patient and physician. The Goldmann device currently is the most widely used instrument for determining intraocular pressures and is supplied by Haag Streit Ag. Ophthalmological Instruments, 3097 Liedefeld, Berne, Switzerland. The Goldmann device uses an applanation tip which is supported on an arm attached to a slit lamp biomicroscope. It is placed in contact with the surface of the cornea with the patient in the seated position. By use of a slit lamp, the technician views the tip-cornea contact area while varying the applied force on the tip to flatten the cornea. When the flattened corneal area coincides with a standard calibration image of the tip, the force measured is recorded. Viewing of the calibration image is enhanced by applying a fluorescein dye to the cornea.
Even though forces of small magnitude are involved, the accuracy of applanation tonometry is excellent. It is recognized as the international standard to which all other measurements are compared. Applanation tonometry by the Goldmann technique is the standard initial screening for intraocular pressure related problems.
This very beneficial tonometry process also carries with it certain recognized disadvantages. It is recognized that bacterial and viral infections have been spread through patient contact with infected tonometers. It is known, for example, that viral and bacterial infections can be spread fomitically. The applanation tip is a carrier of these contaminants. Adenovirus of several strains is readily transmissible by tear or mucus membrane contact. Hepatitis is likewise known to be present in the aqueous secretion of tears and it is likely that inoculation to a second patient is not only possible but probable. The active AIDS virus has been identified in tears as well as in other glandular secretions.
To avoid infection, attempts have been made to sterilize the applanation tip between uses. Sterilization methods have not been entirely successful, in that they are not reliable, especially with viral infections. Second, disinfectant can itself cause problems. Commercially available hydrogen peroxide of the type used with contact lenses has been used; however, there are reported cases of corneal toxicity caused by use of hydrogen peroxide to sterilize an applanation tip. Residues of common disinfectants such as household bleach, isopropyl alcohol or acetone can immediately desiccate and/or injure the corneal epithelium. In addition, repeated sterilization of the sensitive applanation tip will damage the tip.
In applications of community screening and portable tonometry clinics, it is impractical, if not impossible, to perform the time consuming disinfectant process. Thus, there is an urgent need for a practical means to prevent the spread of infection in tonometry testing.